The present invention is directed to a device for drilling undercuts in boreholes in a substrata and includes a housing abutting against a surface of the substrata, a guide sleeve with an eccentric bore parallel to the axis of the guide sleeve, and a bushing supported in the guide sleeve bore so that it can be rotated through a limited arc of about 180.degree. relative to the guide sleeve. A drive shaft with a cutter head is rotatably supported in the bushing. A torsion spring is located between the guide sleeve and the bushing and is stressed by relative rotation between the guide sleeve and bushing. A releasable member affords rotary arrest between the guide sleeve and the housing. An eccentric gear train within a gear train housing includes an internal ring gear connected with the gear train housing and positioned between the drive shaft and the eccentric bushing.
A drilling device is disclosed in DE-P 39 03 889.0 in which the eccentricity of an eccentric bushing corresponds approximately to the eccentricity of a bore in the guide sleeve. In an initial position, these eccentricities cancel one another, whereby the drive shaft is located centrally relative to the guide sleeve. In this position, the drilling device can be introduced into a borehole and, subsequently, the drive shaft can be powered from the outside. The eccentric bushing is turned by an eccentric gear train. Entrainment of the guide sleeve is initially prevented by the releasable arrangement for rotary arrest, so that relative rotation takes places between the eccentric bushing and the guide sleeve due to the stress of the torsion spring. After the eccentric bushing has been displaced through 180.degree., the drive shaft within the bushing has achieved its maximum eccentricity and the cutter head can penetrate into the surface of the borehole. The arrangement providing rotary arrest of the guide sleeve is released, whereby a circumferential undercut is formed as the eccentric bushing continues to rotate. When this process is completed, the guide sleeve moves relative to the eccentric bushing, while the stress in the torsion spring is released. Accordingly, the drive shaft returns to the initial central position with respect to the guide sleeve and the drilling arrangement can be removed from the borehole.
Rotation of the guide sleeve relative to the bushing can be hindered by friction in the borehole with the result that the drive shaft remains in an eccentric position relative to the guide sleeve and the drilling device cannot be removed from the borehole because the cutter head projects radially outwardly from the guide sleeve.